1. Field of Invention
The present invention relates to a bearing arrangement and more particularly, although not exclusively, to a thrust bearing.
2. Related Art
Thrust bearings represent a subset of rotary bearings which are designed to support a rotating member under axial loading. Examples in which such axial loading can occur include shafts in, for example, gas turbine engines, wind turbines or other load-bearing shafts in marine, aerospace or automotive drive systems. The following description proceeds in relation to thrust bearings for gas turbine engines but may be equally applicable to other thrust bearing applications.
In FIG. 1, there is shown an exemplary thrust bearing arrangement in which an engine powerplant 10 drives a gearbox 12 via a rotating shaft 14. The powerplant and gearbox are both mounted to a supporting structure, shown generally at 16. Bearing arrangements 18 and 20 are mounted in a spaced relationship on the shaft 14 in the vicinity of the power plant 10 and the gearbox 12 respectively. The supporting structure may comprise one or a number of connecting members defining a force path between the powerplant 10 and gearbox 12 and hence the associated bearings 18 and 20.
If a moment ‘M’ is applied to the output shaft of the gearbox then the supporting structure will deflect in dependence upon the imposed deflection of the coupling shaft 14. The bearings 18 and 20 as a result become misaligned to a degree dependent on the stiffness of the structure 16, the shaft 14 and upon the magnitude of the applied moment ‘M’.
Single row (thrust) ball bearings are widely used in many applications where there is the requirement to accommodate both axial and radial loads under high or low speeds.
Referring to FIG. 2, when a bending moment ‘M’ is applied to a shaft supported by the bearings, the bending moment experienced by the shaft increases in a linear manner from the bearing 18 which is further from the point of application of the bending moment M to th e bearing 20 which is closest to the point of application. This increase is depicted by the ramped section 22 of bending moment plot 24. When an angular misalignment is applied to the arrangement, a step or jump 26 in the bending moment occurs at bearing 20 as a result of the stiffness in the bearing 20.
The bearing stiffness can be considered a constant for the system and so, if a shaft reduces in diameter and/or if the distance between the bearings is increased, the stiffness of the shaft reduces and the relative impact of the stiffness of the bearing 20 on the system is increased. In an arrangement which has a relatively long, thin and/or flexible shaft, the majority of the stiffness of the system derives from the bearing stiffness.
The ability for conventional single row ball bearings to withstand misalignment is limited. Misalignment will result in higher ball loads within the bearing and a reduction in bearing life. The degree of misalignment considered to be allowable for a particular system depends on a number of factors, including the internal geometry of the bearing design; the physical size of the bearing; the magnitude of applied forces in a radial direction; the duration of the twist on the shaft; and, the stiffness of the shaft and housing. The interaction of these factors will define the maximum angular misalignment possible for the given bearing arrangement.
Furthermore, the mounting of a bearing directly adjacent to the structures which transmit the majority of the thrust load results in a ‘hard’ bearing mounting arrangement. Such an arrangement allows less deflection of the shaft and can increase the gyroscopic moments experienced by the rotating components. The induced unbalanced dynamic response may be worsened as a result of the inherent stiffness in such a system and could lead to a failure event.